1. Field of the Invention
The present invention relates generally to capacitors for high voltage charging and high current discharging rates, and more particularly for transferring high voltage but relatively low energy charges for storage such as high voltage and short duration pulses such as those generated by devices such as piezoelectric elements when subjected to shock loading and rapid discharge of the collected charges.
2. Prior Art
Capacitors are passive two-terminal electrical components that are used to store electrical energy by the generated electrostatic electric fields. In contrast, batteries store chemical energy which is then transformed into electrical energy. The construction of currently available capacitors varies widely, but they all contain at least two electrical conductor elements that are separated by a dielectric (insulator) element. For example, one common capacitor construction consists of metal foils that are separated by a thin layer of insulating film.
In a capacitor, when voltage (a potential difference) is provided across the conductors, a static electric field is developed across the dielectric element between the conductors, causing positive charges to be collected on one conductor element and negative charges on the other conductor element. Energy is thereby stored in the electrostatic field. An ideal capacitor is characterized by a single constant value called capacitance. Capacitance is the ratio of the electric charges on each conductor element to the potential difference between them. The SI unit of capacitance is Farad (F), which is defined as being equal to one Coulomb (C) per Volt (V).
A simple capacitor construction is shown schematically in FIG. 1. It consists of two parallel conductive plates 10 and 11 separated by a dielectric layer 12 with permittivity ∈ (such as air). The capacitor model shown in FIG. 1 may also be used to make qualitative predictions for other capacitive device geometries. The conductive plates 10 and 11 are considered to extend uniformly over an area A and a charge density ±ρ=±Q/A exists on their surface. Assuming that the width of the plates is much greater than their separation d, the electric field near the center of the device will be uniform with the magnitude E=ρ/∈. The voltage is defined as the line integral of the electric field between the plates
  V  =                    ∫        0        d            ⁢              E        ⁢                                  ⁢                  ⅆ          z                      =                            ∫          0          d                ⁢                              ρ            ɛ                    ⁢                                          ⁢                      ⅆ            z                              =                                    ρ            ⁢                                                  ⁢            d                    ɛ                =                  Qd                      ɛ            ⁢                                                  ⁢            A                              Solving this for C=Q/V reveals that capacitance increases with area and decreases with separation
  C  =            ɛ      ⁢                          ⁢      A        d  The capacitance is therefore greatest in devices made with dielectric materials with a high permittivity, large plate 10 and 11 surface area, and small distance d between plates.
As is shown in the schematic of FIG. 1, certain conductive leads 13 are also attached to the conductive plates 10 and 11, through with the capacitor is connected to the intended electrical or electronic circuitry. In the schematic of FIG. 1, the conductive lead is shown to be connected to the top plate 10 at the point 14.
Capacitors deviate from the aforementioned ideal capacitor model in a number of ways. Some of these, such as leakage current and parasitic effects are nearly linear and can be dealt with by adding virtual components to the equivalent circuit of the capacitor. In other cases, such as with breakdown voltage, the effect is non-linear. Other factors such as temperature dependency may also become important. Finally, combined parasitic effects such as inherent inductance, resistance, or dielectric losses can exhibit non-uniform behavior at variable frequencies of operation.
In practice, the dielectric between the plates passes a small amount of leakage current and also has an electric field strength limit, the breakdown voltage. The conductors and leads also introduce generally undesired inductance and resistance.
Above a particular electric field, known as the dielectric strength Eds, the dielectric in a capacitor becomes conductive. The voltage at which this occurs is called the breakdown voltage of the device and is given by the product of the dielectric strength and the separation between the conductorsVbd=Eds dThe maximum energy that can be stored safely in a capacitor is limited by the breakdown voltage.
The breakdown voltage is critically affected by factors such as the geometry of the capacitor conductive parts; sharp edges or points increase the electric field strength at that point and can lead to a local breakdown.
In certain applications such as when collecting charges from piezoelectric elements used in energy harvesting devices, the total amount of available charges for transfer to storage capacitor(s) is relatively low but is high in voltage. In such applications, since the amount of electrical energy generated by the piezoelectric elements is proportional to the square of the generated voltage level, it is highly desirable to design such electrical energy harvesting (generator) devices to operate at high voltage levels. In such applications, the capacitance of the required storage capacitor(s) can be readily determined to ensure that upon transfer of the generated charges, the voltage level of the capacitor is below the capacitor breakdown voltage. However, since the initial voltage of the piezoelectric (or the like) element is significantly higher than the breakdown voltage of the capacitor, appropriate steps are required to be taken to avoid damage to the collection capacitor(s). Examples of such steps and the means of implementing them may include various circuitry and components to lower the voltage below the capacitor breakdown voltage. Such voltage reduction circuitry and other methods developed to date have significant shortcomings for use in energy harvesting and other similar devices in which relatively small amount of electrical charge and high voltage, such as charges generated by a piezoelectric element under shock loading, is to be transferred to a capacitor(s) for storage. Here, by relatively small amount of charges refers to those amounts of charges that once stored in the target capacitor(s) would result in capacitor voltages that are below their breakdown voltages.
It is therefore highly desirable to have capacitors available for storing electrical energy generated by devices such as energy harvesting devices using piezoelectric elements or magnet and coil devices or other similar devices in which the electrical energy is generated as high voltage and relatively low charge pulses. Such capacitors would then allow the energy harvesting (electrical energy generators) to operate at high levels of efficiency by generating significantly larger amount of electrical energy than is otherwise possible at low voltage below capacitor breakdown voltages. For example, a piezoelectric element used in an electrical energy generator of an energy harvesting device or other devices using similar electrical energy generation methods with piezoelectric elements or the like can readily generate relatively small amount of charges at voltages exceeding 200 Volts, which is significantly higher than the breakdown voltage of around 10 Volts or lower for capacitors with high energy density.
It is appreciated by those skilled in the art that capacitors with high energy densities are constructed with very thin dielectric elements and thereby with relatively low breakdown voltages. A goal of the present invention is to provide novel methods and means of making such high energy density capacitors capable of accepting “pulsed” electrical energy charges at high voltages such as those generated by piezoelectric elements of energy harvesting power sources that generate electrical energy by “pulses” of electrical charges at high voltages relative to the breakdown voltages of said capacitors. The electrical energy “pulses”, once charges have distributed over the capacitor conductors, i.e., once a steady state condition has been reached, are considered to generate capacitor voltages that are at or below the breakdown voltages of said capacitors.